1. Field of the Invention
The present invention relates to laser generators for optical fibre transmission systems, and more particularly to the laser emission of solitons using an erbium-doped active fibre with passive mode locking and non-linear polarization switching.
2. Discussion of the Related Art
Under many circumstances in the telecommunications field it is desirable to obtain pulsated laser emissions of a particularly limited duration. Pulses of the above kind having a duration .delta.t&lt;100 ps, commonly referred to as ultrashort pulses, or "solitons" under guided propagation conditions, apply for example in the field of high speed digital telecommunications (up to 10 GBit/s), for making optical instruments, carrying out tests on semiconductor components and also making remote measurements, in topography or in the atmospheric-radar field, for example.
To this end, optical-fibre laser devices using erbium-doped optical fibres are known, in which an active modulating device of the electro-optical type is inserted in an optical path forming a laser cavity so as to enable propagation and amplification of the desired pulses alone. Such devices are commonly referred to as mode-locked devices of the active type, because the modulating device present therein and operated from the exterior acts on the modes generated in the laser, thus enabling amplification as far as a level supporting the laser emission in the selected modes alone. Devices of the above kind are for example described in OPTICS LETTERS vol. 14, No. 22, Nov. 15, 1989, pages 1269-1271 by J. D. Kafka, T. Baer and D. W. Hall, in ELECTRONICS LETTERS, vol. 26, No. 3, Feb. 1, 1990, pages 216-217, by A. Takada and H. Miyazawa, and in PROCEEDINGS OF II TOPICAL MEETING ON OPTICAL AMPLIFIERS, Optical Society of America, 1991, Snowmass Village, Colo., U.S., pages 116-119, by T. Pfeiffer and H. Schmuck (SEL Alcatel Research Centre).
Also known are mode-locked pulse laser generators of the passive type comprising optical-fiber laser devices (a "figure-of-eight" laser) in which a non-linear amplifier loop mirror (NALM) is employed which is connected to a unidirectional loop to make a laser in which mode-locking is generated in the absence of external drive means. In the above devices, the non-linearity of the amplifier loop, when the highest luminous intensities occur, causes a light deviation from one inlet fibre to the other at the unidirectional loop. In this manner, the system exhibits lower losses at higher luminous intensities than at lower luminous intensities, so that only pulses of greater intensity are maintained. This condition forces the laser to operate in a pulsated manner. Devices of this kind are described in ELECTRONICS LETTERS; Vol. 27, No. 6, Mar. 14, 1991, pages 542-543, by D. J. Richardson, R. I. Laming, D. N. Payne, V. Matsas, M. W. Phillips, in ELECTRONICS LETTERS, Vol. 27, No. 9, Apr. 25, 1991, pages 730-732 by the same authors, and in ELECTRONICS LETTERS, Vol. 27, No. 6, Mar. 14, 1991, pages 544-545, by I. N. Duling.
Theoretical foundations concerning mode-locking both of the active and passive type are disclosed, for example, in WAVES AND FIELDS IN OPTOELECTRONICS, by Herman A. Haus, issued in 1984 by Prentice-Hall, Inc., Englewood Cliffs, N.J., pages 254-290. In ELECTRONICS LETTERS, Vol. 27, No. 3, Jan. 31, 1991 pages 229-230, G. J. Cowle and D. N. Payne describe a loop laser device comprising a selective reflector or mirror, a directional coupler, a polarization controller, an erbium-doped optical fibre and an optoisolator, disposed so as to form a resonant cavity. Such a device is specifically provided for continuous operation in a longitudinal mono-mode.